The present invention relates generally to the control of darkness/lightness in a digital image printing system, and more particularly to identification of saturated image structures requiring darkness/lightness modification within a digital image and alteration of such structures to control perceived darkness/lightness of the output image.
Darkness/lightness control is a feature commonly known to be employed in xerographic reproduction systems. Specifically, it is well known to control the darkness/lightness of an output document by altering the xerographic processes employed to generate and deposit an image on a substrate. However, such changes to the xerographic process were often imperfectly applied over an entire document, leading to user dissatisfaction, or were particularly problematic for the xerographic system, often resulting in reduced life of the xerographic components and supplies. More recently however, there have been processes that have been described for accomplishing darkness/lightness control in a digital printing system, wherein control is accomplished by directly controlling the image exposure system rather than by indirectly controlling the amount of marking material deposited upon an image substrate. The present invention takes advantage of the improved reliability and resolution of digital image printing systems to accomplish real-time processing of digital image data to alter image data in accordance with a user-specified darkness/lightness level in a document image.
However, in the past, darkness/lightness digital imaging processes have been directed to binary input, and not to now-expected, gray scale input. Additionally, a substantial portion of the gray scale input will have been previously subjected to antialiasing processes, designed to reduce jaggies through the addition of selectively applied gray pixels. In addition, scanned halftone images and gray halftones from various sources present a similar need. That is, an image structure with some darkened central region may possess a gray edge, and it is desirable to give that image structure a darker or broader appearance.
Heretofore, a number of patents and publications have disclosed bitmap image processing methods, including U.S. Pat. No. 4,460,909, U.S. Pat. No. 4,544,264, and U.S. Pat. No. 4,625,222 to Bassetti; U.S. Pat. No. 5,091,971 to Ward et al.; U.S. Pat. No. 5,282,057 to Mailloux et al.; U.S. Pat. No. 5,359,423 to Loce; U.S. Pat. No. 5,387,985 to Loce et al.; U.S. Pat. No. 5,483,351 to Mailloux et al.; U.S. Pat. No. 5,555,557 to Mailloux. Also noted is U.S. patent application Ser. No. 09/072,122 by Loce et al., entitled, xe2x80x9cMethod and Apparatus for Digital Image Darkness Control using Quantized Fractional Pixelsxe2x80x9d U.S. Pat. No. 5,128,698 to Crawford et al. employs control of the illumination intensity at edge pixels to control placement of image edges.
The book xe2x80x9cEnhancement and Restoration of Digital Documents: Statistical design of nonlinear algorithmsxe2x80x9d by R. Loce and E. Dougherty, SPIE Optical Engineering Press (1997) also describes template-based morphological operations as applied to image bitmaps. The relevant portions of the publication are hereby incorporated by reference.
Crawford, J. L. and C. D. Elzinga, xe2x80x9cImproved Output Quality by Modulating Recording Power,xe2x80x9d SPSE 41st Annual Conference, May 22-26, 1988 Arlington Va., describes logic mask techniques to smooth bit map images while dilating image structures. Power modulation of the laser is employed.
The references cited herein are incorporated by reference for their teachings.
The present invention is directed to a darkness control for adjusting darkness of certain image structures, using morphological processing.
In accordance with the present invention, there is provided a process for control of darkness/lightness in a digital image containing antialiased line art, including: detecting borders within images to be adjusted; adjusting darkness of said detected borders in accordance with desired results; and compacting diffuse gray formed at said borders.
In accordance with another aspect of the invention, detection of borders includes thresholding digital images to be adjusted to remove gray areas, subsequently identifying black/white transitions as borders, and designating as a border pixel any white pixel with a black neighboring pixel after thresholding.
In accordance with still another aspect of the invention, darkness adjustment includes assigning a desired amount of darkness, in the form a gray value onto the detected border pixels.
In accordance with yet another aspect of the invention, said gray compaction includes shifting gray from a first pixel that is not adjacent to a saturated pixel to a second pixel that is adjacent to a saturated pixel; and updating a pixel value in response to said shifting.
The present invention may be applied to similar forms of gray-edged image structures such as halftone dots, and in that setting a change in tone would be perceived. The invention also has particular value in providing a method of adjusting darkness on antialiased line art, so as not to disrupt existing antialiasing. The process is a combination of logical processes that provide a robust system applicable to a large number of cases.